HTTP長連接、短連接使用及測試

概念

HTTP短連接（非持久連接）是指，客戶端和服務端進行一次HTTP請求/響應之后，就關閉連接。所以，下一次的HTTP請求/響應操作就需要重新建立連接。
HTTP長連接（持久連接）是指，客戶端和服務端建立一次連接之后，可以在這條連接上進行多次請求/響應操作。持久連接可以設置過期時間，也可以不設置。

我為什么沒有說HTTP/1.0 默認短連接，HTTP/1.1起，默認長連接呢？因為我第一次看這個說法的時候，以為自己懂了，其實並沒有懂。長短連接操作上有什么區別，有的地方出現的持久連接又是怎么回事？

 

使用設置

這里的設置，我們都以HTTP1.1協議為例子。

設置HTTP短連接

在首部字段中設置Connection:close，則在一次請求/響應之后，就會關閉連接。

設置HTTP長連接，有過期時間

在首部字段中設置Connection:keep-alive 和Keep-Alive: timeout=60，表明連接建立之后，空閑時間超過60秒之后，就會失效。如果在空閑第58秒時，再次使用此連接，則連接仍然有效，使用完之后，重新計數，空閑60秒之后過期。

設置HTTP長連接，無過期時間

在首部字段中只設置Connection:keep-alive，表明連接永久有效。

實現原理

了解怎么設置之后，就開始用起來。然而，問題來了。在請求頭中設置Connection:keep-alive，為什么連接空閑一段時間之后，還是斷開了呢？這是因為connection字段只有服務端設置才有效。

HTTP操作是請求/響應成對出現的，即先有客戶端發出請求，后有服務端處理請求。所以，一次HTTP操作的終點操作在服務端上，關閉也是由服務端發起的。
接下來我們做做測試，以及show code。下面的測試都是使用Spring RestTemplate，封裝apache http client進行的。為方便講解代碼，先說明長連接的情況，最后再對其他形式做測試總結。

客戶端連接失效時間大於服務端失效時間

如下，為請求日志。客戶端設置Connection: Keep-Alive和Keep-Alive: timeout=60， 服務端設置Connection: Keep-Alive和Keep-Alive: timeout=5。

## 客戶端設置有效期為60s [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "POST /adx-api/api/creative/upload HTTP/1.1[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Accept: application/json, application/*+json, text/html, application/json, text/javascript[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Content-Type: application/json;charset=UTF-8[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "User-Agent: Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/31.0.1650.16 Safari/537.36[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Accept-Encoding: gzip,deflate[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Accept-Language: zh-CN[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Connection: keep-alive[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Keep-Alive: timeout=60[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Content-Length: 396[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Host: bizdomain[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "request data" ##服務端設置有效期為5s [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "HTTP/1.1 200 OK[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Date: Wed, 26 Apr 2017 06:07:58 GMT[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Server: Apache-Coyote/1.1[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Content-Type: text/html;charset=utf-8[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Keep-Alive: timeout=5, max=100[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Connection: Keep-Alive[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Transfer-Encoding: chunked[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "63[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "response data"

客戶端設置的有效期大於服務端的，那么實際連接的有效期呢？三分鍾之后再次請求，從連接池中lease連接的時候，提示Connection expired @ Wed Apr 26 14:08:05，即在上一次請求之后的5s失效，說明是服務端的設置生效了。

[2017-04-26 14:11:00 DEBUG] (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection request: [route: {}->http://bizdomain:80][total kept alive: 1; route allocated: 1 of 32; total allocated: 1 of 200] [2017-04-26 14:11:00 DEBUG] (org.apache.http.impl.conn.CPool:?) - Connection [id:2][route:{}->http://bizdomain:80][state:null] expired @ Wed Apr 26 14:08:05 GMT+08:00 2017

源碼分析

通過源代碼了解一下連接失效時間的設置過程。

//org.apache.http.impl.execchain.MainClientExec#execute ...... //從連接池中lease connection final HttpClientConnectionmanagedConn = connRequest.get(timeout > 0 ? timeout : 0, TimeUnit.MILLISECONDS); ...... //將conenction封裝在ConnectionHolder中 final ConnectionHolder connHolder = new ConnectionHolder(this.log, this.connManager, managedConn); ...... // The connection is in or can be brought to a re-usable state. //如果返回值消息頭中connection設置為close，則返回false if (reuseStrategy.keepAlive(response, context)) { // Set the idle duration of this connection //取出response消息頭中，keep-alive的timeout值 final long duration = keepAliveStrategy.getKeepAliveDuration(response, context); if (this.log.isDebugEnabled()) { final String s; if (duration > 0) { s = "for " + duration + " " + TimeUnit.MILLISECONDS; } else { s = "indefinitely"; } this.log.debug("Connection can be kept alive " + s); } //設置失效時間 connHolder.setValidFor(duration, TimeUnit.MILLISECONDS); connHolder.markReusable(); } else { connHolder.markNonReusable(); }

待讀取響應之后，釋放連接，即：connHolder.releaseConnection()。調用org.apache.http.impl.conn.PoolingHttpClientConnectionManager#releaseConnection方法。

    @Override public void releaseConnection(final HttpClientConnection managedConn, final Object state,final long keepalive, final TimeUnit tunit) { Args.notNull(managedConn, "Managed connection"); synchronized (managedConn) { final CPoolEntry entry = CPoolProxy.detach(managedConn); if (entry == null) { return; } final ManagedHttpClientConnection conn = entry.getConnection(); try { if (conn.isOpen()) { final TimeUnit effectiveUnit = tunit != null ? tunit : TimeUnit.MILLISECONDS; entry.setState(state); //設置失效時間 entry.updateExpiry(keepalive, effectiveUnit); } } finally { 。。。。。。 } } } }

然后再下一次HTTP操作，從連接池中獲取連接時

//org.apache.http.impl.conn.PoolingHttpClientConnectionManager#requestConnection調用org.apache.http.pool.AbstractConnPool#lease， //調用getPoolEntryBlocking,調用org.apache.http.impl.conn.CPoolEntry#isExpired @Override public boolean isExpired(final long now) { final boolean expired = super.isExpired(now); if (expired && this.log.isDebugEnabled()) { //日志中看到的內容 this.log.debug("Connection " + this + " expired @ " + new Date(getExpiry())); } return expired; }

綜上，連接的實際有效時間，是根據response的設置來決定的。

其他情況測試

• 客戶端設置Connection: Close

##connection:close請求，kept alive的連接為0 [2017-04-26 13:57:00 DEBUG] (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection request: [route: {}->http://bizdomain:80][total kept alive: 0; route allocated: 0 of 32; total allocated: 0 of 200] [2017-04-26 13:57:00 DEBUG] (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection leased: [id: 0][route: {}->http://bizdomain:80][total kept alive: 0; route allocated: 1 of 32; total allocated: 1 of 200] [2017-04-26 13:57:00 DEBUG] (org.apache.http.impl.execchain.MainClientExec:?) - Opening connection {}->http://bizdomain:80 [2017-04-26 13:57:00 DEBUG] (org.apache.http.impl.conn.DefaultHttpClientConnectionOperator:?) - Connecting to bizdomain/127.0.0.195:80 ## 建立新連接 [2017-04-26 13:57:00 DEBUG] (org.apache.http.impl.conn.DefaultHttpClientConnectionOperator:?) - Connection established 127.0.0.191:49239<->127.0.0.195:80 ## 客戶端設置短連接 [2017-04-26 13:57:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Connection: Close[\r][\n]" ## 服務端返回的也是短連接 [2017-04-26 13:57:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Connection: close[\r][\n]" ##請求完之后，關閉連接 [2017-04-26 13:57:00 DEBUG] (org.apache.http.impl.conn.DefaultManagedHttpClientConnection:?) - http-outgoing-0: Close connection [2017-04-26 13:57:00 DEBUG] (org.apache.http.impl.execchain.MainClientExec:?) - Connection discarded [2017-04-26 13:57:00 DEBUG] (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection released: [id: 0][route: {}->http://bizdomain:80][total kept alive: 0; route allocated: 0 of 32; total allocated: 0 of 200]

如上，當服務端返回Connection: Close時，客戶端接收完響應，便會關閉連接。

• 客戶端設置60s超時，服務端設置5s超時

##Keep-Alive: timeout=60 第一次請求,與connection:close無差別 [2017-04-26 10:57:00 DEBUG] (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection request: [route: {}->http://bizdomain:80][total kept alive: 0; route allocated: 0 of 32; total allocated: 0 of 200] [2017-04-26 10:57:00 DEBUG] (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection leased: [id: 0][route: {}->http://bizdomain:80][total kept alive: 0; route allocated: 1 of 32; total allocated: 1 of 200] [2017-04-26 10:57:00 DEBUG] (org.apache.http.impl.execchain.MainClientExec:?) - Opening connection {}->http://bizdomain:80 ## 客戶端設置超時時間60s [2017-04-26 10:57:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Connection: keep-alive[\r][\n]" [2017-04-26 10:57:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Keep-Alive: timeout=60[\r][\n]" ## 服務端設置超時時間5s [2017-04-26 10:57:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Keep-Alive: timeout=5, max=100[\r][\n]" [2017-04-26 10:57:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Connection: Keep-Alive[\r][\n]" ## 服務端設置生效，連接可以保持5s [2017-04-26 10:57:00 DEBUG] (org.apache.http.impl.execchain.MainClientExec:?) - Connection can be kept alive for 5000 MILLISECONDS [2017-04-26 10:57:00 DEBUG] (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection [id: 0][route: {}->http://bizdomain:80] can be kept alive for 5.0 seconds [2017-04-26 10:57:00 DEBUG] (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection released: [id: 0][route: {}->http://bizdomain:80][total kept alive: 1; route allocated: 1 of 32; total allocated: 1 of 200] ##Keep-Alive: timeout=60 非第一次請求 [2017-04-26 14:11:00 DEBUG] (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection request: [route: {}->http://bizdomain:80][total kept alive: 1; route allocated: 1 of 32; total allocated: 1 of 200] ## 連接在上一次請求結束后5s失效 [2017-04-26 14:11:00 DEBUG] (org.apache.http.impl.conn.CPool:?) - Connection [id:2][route:{}->http://bizdomain:80][state:null] expired @ Wed Apr 26 14:10:05 GMT+08:00 2017 

• 客戶端設置失效時間，服務端設置不失效

## 客戶端設置30s超時 [2017-04-26 17:45:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Connection: keep-alive[\r][\n]" [2017-04-26 17:45:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Keep-Alive: timeout=30[\r][\n]" ## 服務端設置永久連接 [2017-04-26 17:45:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Connection: keep-alive[\r][\n]" ## 連接將一直保持 [2017-04-26 17:45:00 DEBUG] (org.apache.http.impl.execchain.MainClientExec:?) - Connection can be kept alive indefinitely

綜上，http連接保持時間是由服務端的消息頭connection字段和keep-alive字段定的。

在上面前兩種情況，請求的是同一個服務端，那么為什么一個返回的是短連接，一個返回的是長連接呢？這里轉一下 這篇文章的解釋：

不論request還是response的header中包含了值為close的connection，都表明當前正在使用的tcp鏈接在請求處理完畢后會被斷掉。以后client再進行新的請求時就必須創建新的tcp鏈接了。 HTTP Connection的 close設置允許客戶端或服務器中任何一方關閉底層的連接，雙方都會要求在處理請求后關閉它們的TCP連接。

補充

• TCP長短連接
  在網上搜資料的時候，看到很多“HTTP協議的長連接和短連接，實質上是TCP協議的長連接和短連接”。 HTTP和TCP是不同兩層的東西，它們怎么會是一樣的呢？HTTP是請求/響應模式的，就是說我們發一個請求一定要有一個回應。最直觀的就是，瀏覽器上發請求，得不到響應就會一直轉圈圈。 而TCP並不是一定要有響應。大家以前使用socket模擬一個IM聊天，A跟B打完招呼，完全可以不用等待B的回應，就自己關掉連接的。

• TCP keep-alive
  另外還有HTTP協議的keep-alive和TCP的keep-alive含義是有差別的。HTTP的keep-alive是為了維持連接，以便復用連接。通過使用keep-alive機制，可以減少tcp連接建立次數，也意味着可以減少TIME_WAIT狀態連接，以此提高性能和提高httpd服務器的吞吐率(更少的tcp連接意味着更少的系統內核調用,socket的accept()和close()調用)。但是，長時間的tcp連接容易導致系統資源無效占用。配置不當的keep-alive，有時比重復利用連接帶來的損失還更大。

而tcp keep-alive是TCP的一種檢測TCP連接狀況的機制，涉及到三個參數tcp_keepalive_time, tcp_keepalive_intvl, tcp_keepalive_probes。
當網絡兩端建立了TCP連接之后，閑置（雙方沒有任何數據流往來）了tcp_keepalive_time后，服務器內核就會嘗試向客戶端發送偵測包，來判斷TCP連接狀況(有可能客戶端崩潰、強制關閉了應用、主機不可達等等)。如果沒有收到對方的回答(ack包)，則會在 tcp_keepalive_intvl后再次嘗試發送偵測包，直到收到對方的ack。如果一直沒有收到對方的ack,一共會嘗試 tcp_keepalive_probes次。如果嘗試tcp_keepalive_probes,依然沒有收到對方的ack包，則會丟棄該TCP連接。TCP連接默認閑置時間是2小時，一般設置為30分鍾足夠了。參考這里。